With the aging of the population globally, the number of people living with dementia — estimated at around 55 million at present — is only likely to grow. But it may be possible to protect your brain from severe cognitive decline and dementia by making sure that it has sufficient stores of vitamin D.

Researchers at Tufts University’s Jean Mayer Human Nutrition Research Center on Aging looked at levels of vitamin D in postmortem samples of brain tissue from adults who suffered from varying rates of cognitive decline. They found that members of this group with higher levels of vitamin D in their brains had better cognitive function.

Vitamin D was indeed present in brain tissue, and high vitamin D levels in all four regions of the brain correlated with better cognitive function.

“Many studies have implicated dietary or nutritional factors in cognitive performance or function in older adults, including many studies of vitamin D, but all of them are based on either dietary intakes or blood measures of vitamin D,” said lead author, Kyla Shea, an associate professor at the Friedman School of Nutrition Science and Policy at Tufts. “We wanted to know if vitamin D is even present in the brain, and if it is, how those concentrations are linked to cognitive decline.”

Vitamin D supports immune responses, bone maintenance and other functions in the body. It is found in fatty fish like salmon and mackerel, as well as in fortified milk, orange juice and cereal. Exposure to sunlight also provides a dose of vitamin D.

The study relied on brain tissue samples from 209 seniors who had taken part in a long-term study of Alzheimer’s disease that began in 1997, the Rush Memory and Aging Project. In that study, Rush University researchers assessed the cognitive function of the participants, older people with initially no signs of cognitive impairment, as they aged, and analyzed irregularities in their brain tissue after death.

Tufts researchers used the samples to look for vitamin D in four regions of the brain — two associated with changes linked to Alzheimer’s disease, one associated with forms of dementia linked to reduced blood flow, and one region without any known links to cognitive decline related to Alzheimer’s disease or vascular disease.

They found that vitamin D was indeed present in brain tissue, and high vitamin D levels in all four regions of the brain were associated with better cognitive function. But the levels of vitamin D in the brain didn’t appear to correlate with any of the physiological markers associated with Alzheimer’s disease in the brain studied, including amyloid plaque buildup, Lewy body disease, or evidence of chronic or microscopic strokes.

It’s still unclear exactly how vitamin D might affect brain function. Another factor is that vitamin D levels are known to vary in different racial and ethnic populations, and most of the participants in the original Rush cohort were white.

“We now know that vitamin D is present in reasonable amounts in human brains, and it seems to be correlated with less decline in cognitive function,” Shea says. “But we need to do more research to identify the neuropathology that vitamin D is linked to in the brain before we start designing future interventions.” She pointed out that dementia has many causes, including those not yet explored.

Sarah Booth, the senior author of the study and director of the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA) at Tufts, added, “This research reinforces the importance of studying how food and nutrients create resilience to protect the aging brain against diseases such as Alzheimer’s disease and other related dementias.”

Don’t rush out to buy a high potency vitamin D supplement, however. The team warns the recommended dose of vitamin D is 600 IU for people from 1 to 70 years old, and 800 IU for those older. Excessive amounts can cause harm, and have been linked to the risk of falling.

The study is published in Alzheimer’s and Dementia, the Journal of the Alzheimer’s Association.